Water in diesel oil emulsions are nowadays known and used as fuels in diesel internal combustion engines and in heating burners, with the purpose of reducing the presence of harmful pollutants in exhaust gases, such as NOx, SOx, CO and particulate matter (PM) without compromising engine's performance.
The practice of adding water to diesel oil dates back to the early 1900s, and there are many different patented implementations. Depending on the application, the water content may vary from 8% to 35%, with a typical 13% water formulation for use in diesel engines.
There are several benefits in adding water to diesel oil. Water vaporization in the combustion chamber provides for fuel dispersion in the form of very small droplets and increases the contact surface between fuel and air. As a result, combustion is more efficient, homogeneous and with lower thermal peaks, thereby reducing poison emissions such as NOx and PM and other unwanted by-products. Nowadays, emulsion fuel technology is considered as one of the most promising solutions to the conservation of fossil fuels. However, there are very few real emulsions on the market due to their scarce stability, reduced power and excessive costs.
Technically these emulsions, also named “white diesel oils” for their peculiar color, are emulsions of water in diesel oil, i.e., W/O emulsions in which water, in the form of small particles or droplets, is uniformly distributed within diesel oil. In these systems, water is the dispersed internal or discontinuous phase, whereas diesel oil is the dispersion medium also known as the external or continuous phase. To date, many different attempts have been made to put the technology into a widespread commercial use, although they have substantially failed to yield reliable and reproducible performance.
The first emulsions were not satisfactory. In fact, they required modifications of the combustion apparatus, did not guarantee an optimal dispersion of water in diesel oil and were not stable enough over time thus compromising the expected reduction of emissions and the calorific yield of the combustion process.
Furthermore, these old emulsions were rather expensive due to the high cost of the production equipment and of the emulsifying agents.
Emulsions are, by nature, physically unstable that is, when stored they tend over time to separate into two distinct phases or layers, with the aqueous phase at the bottom. The phenomenon of complete and irreversible separation of the dispersed phase (water) is known as coalescence.
When phase separation occurs, for instance in a tank, the fuel may cause a considerable worsening of the performance of the engine and even its permanent damage. In fact, the presence of a bulk aqueous phase may cause seizing of feeding pumps and of fuel injectors. The fuel may fail to ignite when the machine is started or stopped due to the lower combustion temperatures, it may become corrosive and may freeze at lower temperatures.
The Applicant further observed that, since the phase separation gradually proceeds, also the combustion efficiency of the fuel over time varies and its assessment provides for variable values depending on the moment the combustion experiment is performed.
High stability over time and under a wide temperature range—e.g., at least four months under normal storage conditions and between −20° C. and +50° C.—is an essential requirement for a commercial product and in particular it is mandatory under Italian law regulating fuel emulsions.
There have been many attempts to manufacture stable water in diesel oil emulsions but very few reached the market.
System stabilization has been tried mainly by using specific surfactants and/or mechanical means, which allow the emulsion to stay homogeneous over a period sufficiently long to be used without separation of the two immiscible phases.
In the preparation of emulsions, manufacturing conditions are at least as crucial as the selection of proper surfactants. Common methods for preparing emulsions require energy to be added to the system in some form. Energy may be supplied in a variety of ways e.g. trituration, homogenization, shaking, stirring, and heat.
The energy necessary to reach the emulsified state is considerable and is higher when the surface tension of the two phases is very high. In order to lower the energy content of the system—thus increasing its thermodynamic stability—it is important not only to add surfactants, which reduce surface tension, but also to ensure the formation of the highest possible number of colloidal particles and thoroughly disperse them.
Emulsifying agents have been added to prevent coalescence but the development of these agents is still in the fledgling stage and presents several challenges, including possible negative effects on combustion.
Over the years, several studies focused on emulsifying compositions, which afforded a uniform dispersion of water in diesel oil through generation of micelles.
Several surfactants, above a certain concentration value known as the critical micelle concentration (CMC), organizes themselves in supramolecular aggregates named micelles. In apolar solvents, the hydrophilic part of the surfactants is oriented inwards, while the hydrophobic part is oriented outwards. Above the C.M.C., several properties of the system are positively affected: solubility increases, interfacial tension decreases and the overall stability of the emulsion improves.
Over the years, the size of the dispersed phase particles has become smaller and smaller (micro or nano-emulsions). If droplets are sufficiently small, the force of gravity acting on them is lower than thermal fluctuations or subtle mechanical agitation forces. In this case, the emulsion can become stable almost indefinitely.
Emulsifiers were also needed, in higher amounts, in order to incorporate more water, which advantageously provides for a further reduction of pollutants in the emissions.
However, addition of high amount of emulsifiers to fuel emulsions to improve stability and to increase water content, besides being particularly expensive, is not devoid of technical drawbacks.
In fact, emulsifiers may cause the formation of carbonaceous deposits adhering to the inner surface of the combustion chamber and of the injection apparatus. These deposits negatively affect engine operation and need to be continuously removed.
It would be thus desirable to have w/o fuel emulsions characterized by a long term shelf-life and reduced emissions and good engine performance. It would even be more desirable to have w/o fuel emulsions containing low amount of surfactants meanwhile offering at least the same or even better stability, water content, engine out-put and pollutant reduction of previous emulsions comprising higher amount of surfactants.
The Applicant has observed that micro-emulsions containing surfactants at low concentration were mentioned in previous patents but generally were not specifically disclosed.
The Applicant has also realized that prior art emulsion compositions with a low content of emulsifiers either were not able to incorporate water in high amount or had a limited stability. Furthermore, these emulsions very often were significantly less performant than diesel oil fuel.
For instance, document EP0958853 discloses a process for producing emulsions of at least two substantially immiscible fluids, particularly emulsions of a liquid fuel with water. Emulsions actually disclosed comprise 2.0-2.5% of surfactants.
Patent Application WO97/34969 describes an emulsion between water and hydrocarbons, for example a Diesel fuel oil. The emulsions reported in the examples are stabilized by addition of about 1.9-2% of an emulsifying composition made essentially of sorbitan sesquioleate, polyethylene glycol monooleate and an ethoxylated nonylphenol, having HLB between 6 and 8.
Patent Application EP812615A1 describes a process for the production of a stabilized emulsion of diesel oil and water. This process provides the preparation of a first emulsion obtained by mixing diesel oil, water and the surfactant, and then the mixing of the so obtained emulsion with more water to produce the final emulsion. The emulsion is prepared by using a hydrophilic or lipophilic surfactant or a mixture thereof in amounts of about 1.2% with respect to the whole emulsion weight.
Patent Application WO 92/19701 discloses emulsions stabilized by addition of alkanolamides. In the preparation of Example 1 the emulsifier oleamide DEA is used in amount of 0.25% by weight and the emulsion is said to be stable for two hours under static conditions.
There is still the need of surfactant compositions for water in diesel oil micro-emulsions that, even with a lower content of surfactants, are still able to provide very stable fuel micro-emulsions, possibly comprising water also in high amount and with combustion performance comparable with diesel oil fuel alone.
The Applicant has surprisingly found a specific emulsifying composition for water in diesel oil micro-emulsions, composition which provides for homogeneous water in diesel oil fuel micro-emulsions, with excellent shelf-life and engine out-put, even with a high water content, which is also effective when low amount of surfactants are used.